20 - Mybatis學習（6）- 懶加載

1 延時加載

• 延遲加載又叫懶加載，也叫按需加載。也就是說先加載主信息，在需要的時候，再去加載從信息。
• 在mybatis中，resultMap標籤 的association標籤和collection標籤具有延遲加載的功能；

1.1 案例

---

• OrderMapper.xml

<!--懶加載-->
    <resultMap id="orderLazyloadingRslMap" type="orders">
        <id column="id" property="id"></id>
        <result column="number" property="number"></result>
        <result column="createtime" property="createtime"></result>
        <result column="note" property="note"></result>

        <!--配置查詢-->
        <association
                property="user"
                select="com.tzb.mapper.UserMapper.findUserById"
        column="user_id">
        </association>
    </resultMap>
    <select id="findOrderAndUserByLazyLoading" resultMap="orderLazyloadingRslMap">
        SELECT * FROM orders
    </select>

---

• 爲使用懶加載時
• 測試

@Test
    public void test1() throws IOException {
        // 代理
        OrderMapper mapper = session.getMapper(OrderMapper.class);

        List<Orders> orders = mapper.findOrderAndUserByLazyLoading();

        for (Orders order : orders) {
            System.out.println("訂單信息：");
            System.out.println(order);

            System.out.println("訂單所屬用戶：");
            System.out.println(order.getUser());
        }

    }

DEBUG [main] - Setting autocommit to false on JDBC Connection [com.mysql.jdbc.Connection@75e04ee8]
DEBUG [main] - ==>  Preparing: SELECT * FROM orders 
DEBUG [main] - ==> Parameters: 
DEBUG [main] - ====>  Preparing: SELECT * FROM user WHERE id = ?; 
DEBUG [main] - ====> Parameters: 1(Integer)
DEBUG [main] - <====      Total: 1
DEBUG [main] - ====>  Preparing: SELECT * FROM user WHERE id = ?; 
DEBUG [main] - ====> Parameters: 10(Integer)
DEBUG [main] - <====      Total: 1
DEBUG [main] - <==      Total: 3
訂單信息：
Orders{id=3, user_id=null, note='null', number='1000010', createtime=Wed Feb 04 13:22:35 CST 2015}
訂單所屬用戶：
User [id=1, username=王五, sex=2, birthday=null, address=上海]
訂單信息：
Orders{id=4, user_id=null, note='null', number='1000011', createtime=Tue Feb 03 13:22:41 CST 2015}
訂單所屬用戶：
User [id=1, username=王五, sex=2, birthday=null, address=上海]
訂單信息：
Orders{id=5, user_id=null, note='null', number='1000012', createtime=Thu Feb 12 16:13:23 CST 2015}
訂單所屬用戶：
User [id=10, username=張三, sex=1, birthday=Thu Jul 10 00:00:00 CST 2014, address=北京市]
after.......關閉 session
DEBUG [main] - Resetting autocommit to true on JDBC Connection [com.mysql.jdbc.Connection@75e04ee8]
DEBUG [main] - Closing JDBC Connection [com.mysql.jdbc.Connection@75e04ee8]
DEBUG [main] - Returned connection 1977634536 to pool.

Process finished with exit code 0

---

• 配置懶加載
  

DEBUG [main] - Setting autocommit to false on JDBC Connection [com.mysql.jdbc.Connection@62158e60]
DEBUG [main] - ==>  Preparing: SELECT * FROM orders 
DEBUG [main] - ==> Parameters: 
DEBUG [main] - <==      Total: 3
訂單信息：
DEBUG [main] - ==>  Preparing: SELECT * FROM user WHERE id = ?; 
DEBUG [main] - ==> Parameters: 1(Integer)
DEBUG [main] - <==      Total: 1
Orders{id=3, user_id=null, note='null', number='1000010', createtime=Wed Feb 04 13:22:35 CST 2015}
訂單所屬用戶：
User [id=1, username=王五, sex=2, birthday=null, address=上海]
訂單信息：
Orders{id=4, user_id=null, note='null', number='1000011', createtime=Tue Feb 03 13:22:41 CST 2015}
訂單所屬用戶：
User [id=1, username=王五, sex=2, birthday=null, address=上海]
訂單信息：
DEBUG [main] - ==>  Preparing: SELECT * FROM user WHERE id = ?; 
DEBUG [main] - ==> Parameters: 10(Integer)
DEBUG [main] - <==      Total: 1
Orders{id=5, user_id=null, note='null', number='1000012', createtime=Thu Feb 12 16:13:23 CST 2015}
訂單所屬用戶：
User [id=10, username=張三, sex=1, birthday=Thu Jul 10 00:00:00 CST 2014, address=北京市]
after.......關閉 session
DEBUG [main] - Resetting autocommit to true on JDBC Connection [com.mysql.jdbc.Connection@62158e60]
DEBUG [main] - Closing JDBC Connection [com.mysql.jdbc.Connection@62158e60]
DEBUG [main] - Returned connection 1645579872 to pool.

Process finished with exit code 0